Looks like a lot less butchery (i.e., much neater job!) than what I did on my grandson's backpack. (Butchery = Backpacks Upscaled Through Creative High-End Remodeling, per a poster on another forum.)
Now that you've already done the cutting, hee hee: It's a good idea to try on the backpack over all your insulating/cold weather/rain gear before cutting to make sure you left enough strap to fit over them. I hope you did this! As was already stated, you really need the compression straps--but if you did cut them, you can always use elastic cord as a lighter substitute.
Reminds me of an old "Blondie" cartoon in which Blondie asked Dagwood to set up a clothesline. She complains, "You cut the rope too short!" Dagwood replies, "If you cut the rope too short, you can always splice it. If you cut the rope too long, though, there's nothing you can do about it."